The purpose of this study is the determination of the viral proteins, their organization and interactions required for MuLV development. Employing a temperature-sensitive mutant of RMuLV, transmembrane associations of viral envelope and core components were demonstrated prior to budding, although budding appeared at times to occur without viral gp70. This latter phenomenon was confirmed in a cell line (HTG2) of hamster origin which is totally lacking gp70. In addition, the core polyprotein of HTG2 virus, which usually remains uncleaved, was shown to undergo cleavage during incubation with disrupted avian myoblastosis virus. The presence of gp70 in an aberrant form also permits virus budding as seen in B16 melanoma cell lines in which gp70 antigenicity is present in an 80,000-85,000 MW protein(s). Finally, studies were begun to determine the effects of interferon on virus production and glycoprotein expression in Swiss/3T3 cells infected with MoMuLV. Additional studies on VSV-infected L cells after interferon treatment showed progressive loss of glycoprotein, and therefore virus spikes, with increasing (5-30 units/ml) interferon concentrations.